This invention relates generally to methods and apparatus for measuring the quality of a cement to casing bond in a wellbore, and more specifically, to methods and apparatus for measuring the quality of the cement based both vertically and circumferentially.
Cement evaluation logs provide information regarding the presence and quality of the cement distribution around the casing in a wellbore. The objectives of the log are to verify that sufficient cement is present to support the casing and, of primary importance, to confirm that zone(s) of interest are hydraulically isolated. Casing mechanical support does not necessarily require 100 percent circumferential coverage of the casing. However, in order to hydraulically seal the formation as well as the casing, a 100 percent fill of the casing-formation annular region with a nonpermeable material must exist. Acoustic techniques have been used for many years to determine the quality of a cement bond. The early instruments relied on measuring the amplitude of the first compressional wave arrivals, propagating longitudinally from transmitter to receiver. The transmitter and receiver(s) were omnidirectional with an approximate operating frequency of 20 kHz. A relationship was established between cement compressive strength and the amplitude of the longitudinal casing acoustic compressional wave. A transmitter and receiver combination was used to measure the attenuation of the longitudinally propagating casing-borne signal. Typical casing bond logging (CBL) instruments have both a three-ft and five-ft transmitter-receiver spacing. The three-ft transmitter-receiver is used to measure the amplitude of the first compressional arrival (El). The five-ft transmitter-receiver spacing is used to produce a Variable Density Log (VDL) and/or Signature Log, a two-dimensional full-wave presentation of the received signal. The VDL/Signature presentation is used to qualitatively evaluate the cement/formation bond.
However, the amplitude of the acoustic signal between transmitter and receiver depends not only on the attenuation from the axial propagation along the casing, but also on numerous other factors which influence the amplitude of the received signal, such as:
1. Calibration. Typically, the receiver sensitivity is adjusted to provide a fixed response in "free pipe" of given casing size, casing weight, borehole fluid, and pressure. Present calibration techniques cannot eliminate all the variable conditions that are encountered in the well bore.
2. Tool Eccentering. The omnidirectional characteristics of the prior art transducers require good centralization to insure simultaneous first arrival from all azimuths. Small amounts of eccentering (Xl/4 in.) can cause severe signal amplitude attenuation.
3. Temperature and Pressure Effects. At high pressure and temperature there is significant effect on transducer (ceramic and magnetostrictive) response. A 20 to 40 percent change in response can be seen with temperature variations (50.degree.-350.degree. F.) and pressure variations (0-20,000 psi).
4. Acoustic coupling. The acoustic coupling between transducer and casing is affected by the acoustic impedance of the transducer, well bore fluid, and casing. The acoustic impedance of a material is a function of its density and acoustic velocity. Thus the amplitude of the received signal is sensitive to borehole fluid density, viscosity, pressure, and temperature.
5. Microannulus Lack of shear coupling between cement/casing due to a microannulus will cause the amplitude of the first compressional arrival to approach the value of unsupported casing.
6. Presence of Channels. The omnidirectional characteristics of the prior art transducers make it difficult to distinguish high-strength cement with a channel (potential hydraulic communication) from an annular region with a 100 percent fill of a low-strength cement (zonal isolation, no communication).
7. Fast Formation. In fast formations, where the compressional velocity is higher than the plate mode velocity in steel (17,540 ft/sec), it is possible for the formation first arrivals to interfere with the casing first arrival. If this occurs, calibration is no longer valid.
To attempt to overcome these problems, a compensated cement bond log was developed. The compensated cement bond log included an axially-spaced pair of omnidirectional cylindrical transmitters and an axially-spaced pair of omnidirectional receivers spaced intermediate the spaced pair of transmitters. The spatial attenuation rate of the signal between each spaced transmitter and receiver pair was determined in accordance with the following relationships: ##EQU1## where: A.sub.12 =signal amplitude measured at receiver 1 from transmitter 2
A.sub.21 =signal amplitude measured at receiver 2 from transmitter 1 PA1 A.sub.11 =signal amplitude measured at receiver 1 from transmitter 1 PA1 A.sub.22 =signal amplitude measured at receiver 2 from transmitter 2
d=axial spacing between receiver 1 and receiver 2 (ft)
However, since such cement bond tools, even the compensated CBL tools, use omnidirectional cylindrical transmitters and receivers, these instruments average the pressure wave from the full circumference of the casing and such instruments have difficulty determining the difference between cement of low compressive strength and "channels" in the cement.
Another class of prior art radial cement bonding instruments use acoustic pulse-echo techniques, in which eight ultrasonic transducers are arranged 45.degree. from each other in a circumferential helical path around the tool housing to encompass a 360.degree. scan. The pulse-echo transducers act as both transmitter and receiver, each transducer emitting a short pulse of ultrasonic energy and then receiving the echo from the casing. This arrangement provides eight focused measurements with very fine resolution. The presence of cement behind the casing is detected as a rapid decay of the casing resonance, while a lack of cement gives a long resonant decay as is well known in the art. However, several shortcomings are inherent in the pulse-echo technique due to the ambiguity of interpretation due to conditions that exist in the well bore:
1. Acoustic Boundaries. The pulse-echo tool responds to not only the casing/cement boundary but, in certain conditions, responds to acoustic boundaries within and beyond the cement.
2. Gas sensitivity. The presence of gas bubbles within the cement sheath or a gas-filled microannulus causes the pulse-echo type measurement to respond as if there was no cement/casing bond.
3. Mud Weight Sensitivity. The rate at which the casing resonance signal decays is a function of the acoustic impedance contrast of the medium behind the casing, and also the medium inside the casing. Typically, the acoustic impedance contrast of a high compressive strength cement and mud is high and the mud effects are insignificant. But as mud weight increases, the contrast of free pipe to bonded pipe decreases rapidly. This is specially true with low compressive strength cements.
4. Tool Eccentering. When the transducer is moved away from the center axis so that the ultrasonic beam is not normal to the casing wall, the signal is reflected away from the transducer.
5. Borehole Coverage. The amount of circumferential coverage decreases with increasing casing size. In a seven-in. casing one can expect approximately 42 percent of the casing to be measured. This drops to 30 percent in the case of a 95/8 in. casing (assuming a spot size of one in. at 500 kHz).
Accordingly, a new radial cement bond instrument has been developed to measure the quality of cement to casing bond. The instrument is capable of measuring the quality of the cement bond both vertically and circumferentially. Four arrays of sectored transmitters and receivers arranged longitudinally along the instrument body provide a compensated radial bond measurement with high azimuthal resolution. The sectored transmitter excites both a compressional wave and a shear wave in the casing. Unlike the standard CBL measurement, the beam pattern of the transmitter is focused in a radial direction. The sectored receiver provides additional directivity for enhanced detection of channels in cement.